CCITT Recommendation G.726 is a widely used, early speech coding standards for telephony. Recently in digital and packet communication systems, packet loss handling mechanism has become very common in the current communication scenarios using VOIP (voice over Internet Protocol) and other packet networks. But the current CCITT Recommendation G.726 does not support any mechanism for packet loss recovery. Thus quality goes down in case of packet loss with bad artifacts and glitches in the speech. These glitches and artifacts are hard to compensate in any subsequent packet loss algorithm and system such as G.711. So there is need to minimize these glitches for proper functioning of a G.726 codec in packet loss scenarios.
In a CCITT Recommendation G.726 system the encoder and decoder states are coupled. During packet loss, the encoder and decoder lose their ability to track states. In addition the tone detector is somewhat ad-hoc and further deteriorates the state tracking ability of the decoder. For tone detection, the predictor poles and zeros are set to zero values. This tone detection also detects the false tones in the normal speech signals. Thus a frame loss makes it very difficult for the decoder to track the encoder because the tone detector would set the predictor poles and zeros to zero values. In this state, the codec output exhibits glitch artifacts in the output speech.
A G.726 codec is Adaptive Differential Pulse Code Modulation (ADPCM) based and operates at 16, 24, 32 or 40 K bits/sec. The codec converts 64 K bits A-law or μ-law pulse code modulated (PCM) channels to and from a 16, 24, 32 or 40 K bits/sec channels using ADPCM transcoding. The heart of the codec is the sign-sign (SS) and leaky LMS algorithm.